Add toggle and what's new icons

[nt1m]

See issues #63 and #62.

[nt1m]

@bwinton Could you please review this ? These icons are taken from mozilla-central:

• https://searchfox.org/mozilla-central/source/browser/themes/shared/icons/toggle.svg (designed by Eric Pang in https://bugzilla.mozilla.org/show_bug.cgi?id=1533862#c5 , currently in Nightly's about:config refresh)
• https://searchfox.org/mozilla-central/source/browser/themes/shared/icons/whatsnew.svg (not sure who designed it, but this is a pretty recent icon that's shipping in Firefox)

[bwinton]

Asking Shorlander for the design review.

[ariasuni]

The two issues mentioned in description of the PR haven’t been closed.